Status signal output

ABSTRACT

A data bus subscriber connected to a local bus, particularly a ring bus. The data bus subscriber has a status signal input for receiving a first status signal value from a downstream data bus subscriber or a terminator, a status signal output for providing a second status signal value to an upstream data bus subscriber or to a local bus master, wherein the data bus subscriber is adapted to provide the second status signal value based on a logical link of a communication readiness of the data bus subscriber and the first status signal value. The invention further relates to a corresponding method and a local bus.

This nonprovisional application is a continuation of InternationalApplication No. PCT/EP2018/062962, which was filed on May 17, 2018, andwhich claims priority to German Patent Application No. 10 2017 208836.2, which was filed in Germany on May 24, 2017, and which are bothherein incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a data bus subscriber for use on alocal bus, in particular a data bus subscriber with a status signaloutput for checking the completeness and operability of the local bus.

Description of the Background Art

Data bus subscribers are mostly used in automation systems.

Automation systems are used in particular for the control of industrialinstallations, buildings and means of transport. For the control of anautomation system, usually several sensors and actuators are necessary.These monitor and control the process performed by the installation. Thedifferent sensors and actuators of an automation system are oftenreferred to as automation devices.

These automation devices can either be connected directly to a controlunit of the automation system or can first be connected to input andoutput modules, which are often referred to as I/O modules. These can inturn be connected directly to the control unit. The automation devicescan either be integrated directly in the I/O modules or can be connectedto them via cable or wirelessly.

The control of an automation system is usually accomplished with thehelp of one or more programmable logic controllers, PLC. The PLCs can bearranged hierarchically or decentrally in an automation system. Thereare different performance levels for the PLCs, so that they can takeover different types of controls and regulating techniques depending onthe computing and storage capacity. In the simplest case, a PLC hasinputs, outputs, an operating system (firmware) and an interface viawhich a user program can be loaded. The user program defines how theoutputs are to be switched as a function of the inputs. The inputs andoutputs can be connected to the automation devices and/or the I/Omodules and the process carried out by the automation system can bemonitored or controlled by the logic stored in the user program. In thiscase, the monitoring of the process is accomplished by the sensors andthe control of the process by the actuators. The control unit can alsobe referred to as a central controller or central unit and assumescontrol of at least one automation device or I/O module connected to thecontrol unit.

However, direct connection of the automation devices to the at least onecontrol unit or of the I/O modules to the at least one control unit inthe form of a parallel wiring, i.e., when one line is routed from eachautomation device or each I/O module to the higher-level control unit,is very expensive. Especially in an increasing degree of automation inautomation systems, the cabling effort increases with parallel wiring.This is associated with great cost in the design, installation,commissioning and maintenance.

For this reason, automation systems generally use bus systems today, bymeans of which the automation devices or the I/O modules can beconnected to the control unit. Such subscribers of a bus system are alsoreferred to as bus subscribers. Because data is exchanged on the bussystem, bus subscribers are also often referred to as data bussubscribers. In order to further simplify the connection of theindividual automation devices or the I/O modules to the bus system,nowadays individual groups of automation devices or I/O modules areinitially interconnected to a local bus system with the help of aspecialized local bus, and then at least one subscriber of this localbus is connected to the bus system, which is connected to the controlunit. In this case, the local bus system may differ from the bus system,which is used to establish the connection with the control unit.

The subscriber of a group of local bus subscribers connected to the bussystem of the control unit is often referred to as a local bus master.Alternatively, the term header of the local bus system is also used. Incomparison to other local bus subscribers, this local bus master maycontain further logic, circuits or functionalities which are necessaryfor connection to the bus system of the control unit. Also, the localbus master itself may include a PLC. This subscriber can also have logicand circuits for conversion between the two bus systems. The local busmaster can therefore also be designed as a gateway or bus converter andit ensures conversion of the data provided in the format of one of thebus systems to the format of the local bus system and vice versa. Inmost cases, however, not mandatory, the local bus master is specializedin the connection of the local bus to the higher-level bus.

The local buses that are used are usually tailored to the specificoperational requirements of the automation devices or I/O modules ortake into account their special hardware design. The groups ofautomation devices or I/O modules of the local bus system usually form asubgroup of the automation system for performing a specific task in theprocess performed by the automation system. The data exchanged on thebuses for the process is also often referred to as local bus data orprocess data, because this data includes information for regulating orcontrolling the process executed by the automation system. This processdata may include, among other things, measurement data, control data,status data and/or other information. Depending on the bus protocolused, this process data can precede (header) or be appended to (tail)other data. This other data may include information regarding theprocess data or include information with respect to internalcommunication on the local bus. Here, a variety of different informationis known, which can precede or be added to the process data according tothe bus protocol used.

A ring bus is a specialized form of a local bus, as known for examplefrom U.S. Pat. No. 5,472,347 A. In a ring bus, the data bus subscribers,for example the automation devices or I/O modules, are each connected tothe data bus subscribers directly adjacent to them and data is forwardedin succession from one to the other data bus subscriber. Thus, not alldata bus subscribers are sent the data at the same time, but insuccession, wherein a data bus subscriber receives data from itsupstream data bus subscriber and forwards data to his downstream databus subscriber. Between receiving the data and forwarding, the data bussubscriber can process the received data. When the data has reached thelast data bus subscriber in the series, the data from the last data bussubscriber is returned back successively to the first data bussubscriber. The return can be performed either through all data bussubscribers or past them by means of a bypass line. The ring bus thushas a downflow and an upward flow of data. The data in a ring bus isoften not transferred in the form of addressed data packets, but insteadin the form of cycle frames that pass through all data bus subscribers.Cycle frames are a cyclically repeating sequence of telegrams/datapackets. Several telegrams/data packets can be present in a cycle frame.Cycle frames are sent cyclically, for example, at a fixed time intervalfrom the local bus master to the data bus subscribers. These cycleframes usually contain a header, an information part and a checksumpart. The information part can thereby carry the telegrams/data packetswhich themselves can carry process data and this part can accordinglyalso be referred to as a process data part. In this process data part,the process data are arranged such that due to the position of theprocess data within the process data part in the cycle frame, forexample within an associated contiguous data block, the data bussubscribers can recognize those process data that are addressed to thedata bus subscriber. Process data addressed to the data bus subscriberare those process data which are suitable for a data bus subscriber tocontrol, regulate or perform an evaluation, in particular to collectmeasurement data and to set output values at outputs of said data bussubscriber.

In a ring bus, the cycle frame is routed from one data bus subscriber tothe other. At any given time, a data bus subscriber always receives onlypart of the cycle frame from its upstream data bus subscriber. When thedata contained in this part has been processed by the data bussubscriber, the part is forwarded to the downstream data bus subscriberand at the same time, a new part of the cycle frame is received by theupstream data bus subscriber. In this way, all parts of the cycle framesequentially pass through all the data bus subscribers.

In modern bus systems, it is possible that data bus subscribers can beadded to the bus or can be removed from this, even during the actualoperation of the bus. For the correct functioning of the bus, it isnecessary that the unit controlling or driving the bus, i.e., the localbus master or the PLC, is informed at all times that the bus system iscomplete and that all data bus subscribers are ready to communicate.

For this purpose, a status signal is routed through all data bussubscribers arranged on the bus, so that, for example, a logical one(high state) or a logical zero (low state) indicates the completenessand/or communication readiness of all data bus subscribers arranged onthe bus.

However, the known bus systems and the data bus subscribers connectedthereto usually have the disadvantage that the status signal isautomatically set to a specific defined state immediately when the databus subscriber is added. However, this can lead to invalid statesbecause, for example, the state is already being changed without acorresponding condition having occurred. In this case, therefore, thelocal bus master or the PLC have no way of ascertaining from the statechange alone whether the corresponding condition has already occurred,for example whether the added data bus subscriber is already ready tocommunicate.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to avoid invalidstate messages.

This object is achieved with a data bus subscriber, a method foroperating a data bus subscriber and a local bus according to anexemplary embodiment.

The data bus subscriber according to an exemplary embodiment of theinvention is connected to a local bus, in particular a ring bus, and hasa status signal input for receiving a status signal from a downstreamdata bus subscriber or a termination unit. The status signal has a firststatus signal value. In accordance with its reception at the statussignal input, this first status signal value may also be called inputstatus signal value, input status value, or input value. The statussignal value can originate from a downstream data bus subscriber, thatis to say a data bus subscriber which directly or indirectly physicallyfollows the data bus subscriber on the local bus. A data bus subscriberis said to be downstream, for example, when it is arranged at a positionon the local bus which follows the position of the data bus subscriber.Alternatively, the first status signal value may also originate from atermination unit. A termination unit is connected to the last data bussubscriber in the local bus and provides a certain status signal valueat a status signal output of the termination unit. Regardless of whetherthe status signal value is provided from a downstream data bussubscriber or a termination unit, this status signal value can bereferred to as an output signal status value, output status value, oroutput value, because it is present at the status signal output of thetermination unit or at the data bus subscriber. The status signal valuemay assume a discrete status signal value, for example, this statussignal value may assume a high state (logical one) or a low state(logical zero).

The data bus subscriber also can have a status signal output forproviding a status signal to an upstream data bus subscriber or to alocal bus master. The status signal may have a second status signalvalue. In accordance with the output at the status signal output, thissecond status signal value may also be referred to as an output statussignal value, output status value, or output value. The upstream databus subscriber can be a data bus subscriber that physically orindirectly precedes the transmitting data bus subscriber in the localbus, or can be the local bus master, which represents the first data bussubscriber in the local bus.

A data bus subscriber or the local bus master can be upstream, forexample, when it is arranged at a position on the local bus whichprecedes the position of the data bus subscriber. The data bussubscriber can be adapted to provide the second status signal value ofthe provided status signal based on a logical link of a communicationreadiness of the data bus subscriber and the first status signal. If,for example, a status signal which has a first status signal valueindicating that the downstream data bus subscriber is ready tocommunicate is present at the status signal input of the data bussubscriber, the data bus subscriber can output this first status signalvalue at its status signal output as the second output status signalvalue as soon as the data bus subscriber itself is ready to communicate.In this case, therefore, the first and second status signal valuecorrespond. If the first status signal value at the status signal inputindicates that the downstream data bus subscriber is not yet ready tocommunicate, the second status signal value cannot be changed to thefirst status signal value even if the received data bus subscriber isready to communicate until the first status signal value also indicatesthat the downstream data bus subscriber is ready to communicate. Thefirst and the second status signal values may assume discrete states,and a specific defined discrete state may indicate that a data bussubscriber is ready to communicate. This means that if the status signalvalue received via the status signal input can be considered a logicalstate, that is, a logical one or a logical zero, and the communicationreadiness can also be expressed as a logical one or logical zero, thenonly a corresponding status signal value is provided at the statussignal output of the data bus subscriber, provided the logical statescorrespond. That is, only if the first status signal value indicatesthat the downstream data bus subscriber is ready to communicate and thedata bus subscriber itself is ready to communicate is the second statussignal value changed to the first status signal value. The behavior canbe created with an AND link. The communication readiness can bedisplayed with a logical one and the first status signal value can benegated, so that if both are at a logical one state, the AND link alsoindicates a logical one state. This link value is then negated again andoutput as the second status signal value. Accordingly, the logical linkcan be arranged such that the first status signal value and the secondstatus signal value assume a same discrete state when the data bussubscriber is ready to communicate. In this case, the logical link canbe set up to set the first status signal value to a fixed state if thedata bus subscriber is not yet ready to communicate, or to otherwisechange the state. In other words, it can also be said that the secondstatus signal value, which is specified by the termination unit andwhich indicates the communication readiness, is looped from thetermination unit to the local bus master through the local bus,depending on the communication readiness of the respective data bussubscribers. It can also be said that there is a status signal linebetween the termination unit and the local bus master, wherein thisstatus signal line is only complete or assumes the status signal valueindicating communication readiness when all the data bus subscribers areready to communicate. Until all data bus subscribers are ready forcommunication, another state can be present at the signal line on thelocal bus master. It can also be said that the status signal linebetween the termination unit and the local bus master is switched insections to one status signal value, which indicates the communicationreadiness of the individual data bus subscribers. In this case, thisstatus signal value corresponds to the one which is specified by thetermination unit. But it can also be that no termination unit isconnected to the last data bus subscriber and that the last data bussubscriber assumes the functionality of the termination unit. Forexample, the last data bus subscriber who determines that no terminationunit is connected, because for example no connection to the statussignal input was made, can output a corresponding first status signalvalue at its status signal output even if no second status signal valueis present at the status signal input. In this case, it is onlynecessary to ensure that the data bus subscriber is aware that itself isthe last data bus subscriber and that the absence of the first statussignal value is based on the position of the data bus subscriber and noton an error of the downstream data bus subscriber. In a furtheralternative, it is also conceivable that the last data bus subscriberhas no status signal input, but only a status signal output. Thisparticular data bus subscriber is thus always the last data bussubscriber in a local bus.

With the data bus subscriber according to the invention and theconditional change of a status signal value on a status signal line,invalid information about the state is avoided because the status signalvalue applied to the local bus master is only changed to a value whichindicates the communication readiness of the data bus subscribers whenall the data bus subscribers are ready to communicate.

The first and second status signal values can assume the same discretestate when the data bus subscriber and the downstream data bussubscriber are ready to communicate. It may also be said that the statussignal at the status signal input and status signal output of the databus subscriber assumes the same status signal value when the data bussubscriber and the downstream data bus subscribers are ready tocommunicate. That is to say, the status signal value at the statussignal input of a data bus subscriber indicates whether the downstreamdata bus subscriber or the downstream data bus subscribers are ready tocommunicate, whereas the status signal value at the status signal outputof the data bus subscriber indicates whether the data bus subscriber isalso ready to communicate. If the first and the second status signalvalue assume the same value, then not only is the data bus subscriberready to communicate but also, inevitably, the downstream data bussubscribers. The logical link of the first status signal value (SSW1) atthe status signal input with the communication readiness (KB) of thedata bus subscriber to generate the second status signal value (SSW2) atthe status signal output is summarized in Table 1, wherein “T” standsfor “true” and “F” stands for “false”. These values are thereforeBoolean values that can only assume two states. Instead of true andfalse, the logical states high and low or one and zero can also be used,which can be linked via logical operators of the Boolean algebraaccording to Table 1.

TABLE 1 Logical link of the first status signal value and thecommunication readiness SSW₁ KB SSW₂ F F F F T F T F F T T T

Thus, a conditional status signal line is established which, whencompleteness has been achieved, indicates that all the data bussubscribers are ready to communicate, because all of the data bussubscribers of the local bus have the same value present at the statussignal input as at the status signal output; as shown in Table 1, allstatus signal outputs then have the same logical value. If there is aninterruption, then the status signal value on a data bus subscriberchanges at its status signal input, causing a change in the statussignal output. This means that all data bus subscribers locateddownstream of the interruption will change their status signal values sothat it can be determined that the local bus is no longer withoutinterruption.

The status signal input and the status signal output can be physicallyseparated from other interfaces of the data bus subscriber. In thiscase, the status signal line defined via the status signal inputs andstatus signal outputs forms a line between the termination unit or thelast data bus subscriber and local bus master, which is separate fromthe local bus and is not part of the local bus. That is to say, thestatus signal value is not sent via the local bus. The local bus remainsaccordingly free of this information and can be used in advance totransport data to data bus subscribers that are already ready tocommunicate.

The data bus subscriber is ready to communicate as soon as the data bussubscriber is clock-synchronized to the local bus. Alternatively, oradditionally, it may also be a condition that a processing unit of thedata bus subscriber can process data received via the local bus. In thiscase, the data can be received, for example, in a telegram/data packetof a cycle frame. The cycle frame passes through the data bussubscribers unit-by-unit, piecemeal, or in parts, for example, in partsor symbols of 8 bits. That is, at any given time, a data bus subscriberwill only ever have a part of the cycle frame. After a predeterminedtime, the part of the cycle frame available to the data bus subscriberis forwarded to the downstream data bus subscriber and another part ofthe cycle frame is received by the upstream data bus subscriber. Forthis purpose, the data bus subscribers must have uniform timing orclocking to which they are synchronized so that the receiving andforwarding of the portions of the cycle frame always happens at the sametime. That is, only when the data bus subscribers are clock-synchronizedare they also ready to communicate. The clock synchronicity of the databus subscribers can be displayed by setting a register value/a flag. Inaddition, it may still be necessary for the display of communicationreadiness that the data bus subscribers cannot only receive and forwardparts of the cycle frame, but that they are also capable of processingthe parts of the cycle frame. This means that the processing unitlocated in the data bus subscriber is initialized and can process theparts of the cycle frame. For example, if the data bus subscriberdetects a first bit pattern transmitted only once in a cycle frame ortelegram/data packet, which signals the start of the cycle frame or atelegram/data packet in a cycle frame or recognizes other signaled data,then the processing unit can be ready to process data from thetelegram/data packet or the cycle frame and accordingly, communicationreadiness can be signaled. In this case, the processing unit may be anarithmetic logic unit, an arithmetic circuit, a microcontroller, oranother digital logic circuit, which is formed as a part of asemiconductor chip and is implemented in an integrated circuit (ASIC),or in a field programmable (logic) gate array (FPGA), or in anotherprogrammable circuit (PLD), or in a discrete gate or transistor logic.If the processing unit is not yet initialized, that is, if it is not yetable to carry out processing, the data can be guided past the processingunit, for example, by means of a switchable bypass line. If theprocessing unit is able to process the data, i.e., to process thereceived parts of the cycle frame, then the part of the telegram/datapacket can be routed through the processing unit, i.e., in this case,the bypass line is no longer needed. The data bus subscriber can have atleast one switch which switches over in a controlled manner between thebypass line and the processing unit as soon as the processing unit isable to process parts of the cycle frame. This switch may be anelectronic switch that can be switched by a signal output from theprocessing unit itself and that switches between the bypass line and theprocessing unit. As soon as the processing unit is ready to processdata, the data bus subscriber is ready to communicate. The data bussubscriber in this case may be able to change the second status signalvalue as a function of the first status signal value.

The data bus subscriber can comprise a controller for changing aresistance value at the status signal output, wherein a change of theresistance value causes a change of the status signal value, or, atransfer of the second status signal value to the upstream data bussubscriber is only possible by the change in the resistance value. Thestatus signal input of a data bus subscriber thereby represents a firstresistance value, for example via a pull-up resistor which keeps thestatus signal input at a certain voltage level. Only when the resistanceat the status signal output of the downstream data bus subscriberchanges does the status signal input take on the status signal value ofthe downstream data bus subscriber. As a result, a targeted statussignal value transfer is possible without there being any periods inwhich indeterminate status messages exist because at any given time, astate for the status signal input is defined. Only when all statussignal outputs have been changed to corresponding resistance values forforwarding the status signal values does the local bus master receivethe status signal and can determine that the local bus is complete andthat all data bus subscribers connected to the local bus are ready forcommunication. This means that only when all data bus subscribers in thelocal bus have changed their resistance values of the status signaloutputs is the status signal present from the last data bus subscriberor the termination unit up to the local bus master. If an error occursin a data bus subscriber, the status signal is no longer present fromthe last data bus subscriber or the termination unit to the local busmaster. In this case, there is an interruption of the status signalline. Even if the data bus subscribers downstream of the interruptingdata bus subscriber pass the status signal or set all their statussignal outputs to a predefined status, and the status signal outputsassume resistance values which allow for the status signal to passthrough, the status signal is stopped at the interrupting data bussubscriber. Namely, this interrupting data bus subscriber does not setits status signal output to a predetermined state and the status signaloutput does not assume a specific resistance value, so that the upstreamdata bus subscriber cannot receive the status signal and therefore doesnot change its own status signal output. That is, starting from theinterrupting data bus subscriber, the status signal outputs are notchanged with respect to the provided state and the resistance value.This makes it easy to check whether a local bus is complete and whetherthe data bus subscribers are all ready to communicate.

The controller can be adapted to change the resistance value of thestatus signal output from a first value to a second value. For example,no status signal is transmitted at the first value and the status signalis transmitted at the second value. In this case, the amount of changein the value of the resistance of the status signal output is dependenton the value of the resistance of the status signal input of theupstream data bus subscriber. For example, the first value is greaterthan the resistance value of the status signal input of the upstreamdata bus subscriber and the second value is less than the resistancevalue of the status signal input of the upstream data bus subscriber, sothat the status signal can be transmitted.

The data bus subscriber can have a pull-up resistor at the status signalinput. This ensures that the status signal input is kept at a certainstate, for example a logical one, if no status signal output of adownstream data bus subscriber is connected. Only when a downstream databus subscriber is connected and the resistance value of the statussignal output of the downstream data bus subscriber is changed incomparison to the pull-up resistor does the status of the status signalinput change, namely to the one of the status signal output of thedownstream data bus subscriber. The data bus subscriber cancorrespondingly change the resistance of its status signal output if thedata bus subscriber is ready to communicate and the downstream data bussubscriber is also ready to communicate. In the upstream data bussubscriber, the change in the value of the status signal output thencauses the pull-up resistor to not indicate the logical state generatedby the pull-up resistor at the status signal input, but instead show thestatus of the data bus subscriber status output, and so on. For example,the pull-up resistor at the status signal input of the data bussubscriber may be configured to maintain the state of the status signalinput at a logical one as long as the status signal output of thedownstream data bus subscriber is high impedance as compared to thepull-up resistor. High impedance can also mean that no data bussubscriber is connected. Only when a data bus subscriber is connected,and the latter changes the resistance value of the status signal outputwith the aid of the controller to a value which is smaller than thepull-up resistor, does the state of the status signal input of the databus subscriber change. In this case, the local bus master can concludefrom a state change at its status signal input that the local bus iscomplete and/or that all data bus subscribers are ready to communicate.The person skilled in the art is aware that the data bus subscriber canalso have a pull-down resistor instead of the pull-up resistor and thatthe logic discussed above in this case thus negates, i.e. reverses.

The controller can be configured in the data bus subscriberindependently of a processing unit. The change in the resistance valueof the status signal output can thus be done independently of the statusof a processing unit present in the data bus subscriber. The controllermay for example be designed to monitor the processing unit and when thecontroller determines the readiness of the processing unit, thecontroller may change the resistance value of the status signal outputaccordingly to signal that the data bus subscriber is ready tocommunicate.

The controller can be embodied as part of a processing unit arranged inthe data bus subscriber. The change in the resistance value of thestatus signal output is thus dependent on the operational readiness ofthe processing unit itself.

The data bus subscriber can be adapted to be arranged on the local busduring the operation of the local bus. For this purpose, the statussignal output can be connected to a status signal input of an upstreamdata bus subscriber and the status signal input can be connected to astatus signal output of the downstream data bus subscriber. Only whenthe newly added data bus subscriber is ready to communicate can thecontroller in the data bus subscriber change the resistance value at thestatus signal output so as to provide the status signal present at itsstatus signal input—provided that the downstream data bus subscribersare already ready to communicate and a status signal is present at thestatus signal input—to the status signal output and forward it to thestatus signal input of the upstream data bus subscriber.

The object of the invention is also achieved by a method for operating adata bus subscriber connected to a local bus, in particular a ring bus.In this case, the method according to the invention comprises receivinga first status signal value from a downstream data bus subscriber orfrom a termination unit at the data bus subscriber, providing a secondstatus signal value to an upstream data bus subscriber or a local busmaster by the data bus subscriber, wherein the providing includeslogical linkage of communication readiness of the data bus subscriberand of the first status signal value for providing the second statussignal value.

The object of the invention is also achieved by a local bus, wherein thelocal bus has at least one data bus subscriber with a status signalinput and a status signal output. According to the invention, the localbus furthermore has a termination unit which has at least one statussignal output. The status signal output of the termination unit and thestatus signal input of the data bus subscriber are connected, and thetermination unit is adapted to provide at its status signal output afirst status signal value which is received at the status signal inputof the data bus subscriber. The data bus subscriber is adapted toprovide a second status signal value at the status signal output basedon the communication readiness of the data bus subscriber itself and thefirst status signal value received at the status signal input of thedata bus subscriber indicating the communication readiness of thedownstream data bus subscribers. The termination unit mentioned here canalso be a further data bus subscriber, which is arranged as the lastdata bus subscriber in the local bus and which assumes the functionalityof the termination unit. This means that the last data bus subscriberprovides a status signal value at its status signal output whichindicates that the data bus subscriber is ready to communicate, even ifno status signal value is present at the status signal input of the lastdata bus subscriber.

The local bus can have a local bus master which has a status signalinput which is adapted to receive the second status signal valueprovided at the status signal output of the at least one data bussubscriber. The local bus master is able to determine the completenessof the local bus when the status signal value changes. In this case, thelocal bus master is aware that the data bus subscribers connected to thelocal bus are all ready to communicate. In order for the local busmaster to know the identity of the data bus subscribers connected to thelocal bus, the latter can also send one or more telegrams/data packetsin a cycle frame to the data bus subscribers after the status signalvalue has been changed, but also prior thereto, in order to identify thedata bus subscribers. The local bus master can be set up to count anumber of communication-ready data bus subscribers and to send atelegram/data packet to retrieve information for each counted data bussubscriber that is ready to communicate. In this case, the local busmaster can be adapted to send a single telegram/data packet to all databus subscribers for counting the communication-ready data bussubscribers in an order, i.e., the local bus master first transmits asingle telegram/data packet to the local bus. Upon receipt of theindividual telegram/data packet, each data bus subscriber already readyto communicate can change the counter contained in the telegram/datapacket, for example, increment or decrement the latter before thetelegram/data packet or parts of the telegram/data packet are sent tothe next data bus subscriber. Data bus subscribers not yet ready tocommunicate only forward the telegram/data packet without changing thefirst counter. The last data bus subscriber returns the telegram/datapacket to the local bus master. Based on the value of the counter, thelocal bus master then knows how many data bus subscribers of the localbus are already ready to communicate. The local bus master can be set upto repeat the transmission of the individual telegram/data packet forcounting the communication-ready data bus subscribers as long as astatus of the second status signal value at the status signal input ofthe local bus master does not correspond to the status of the firststatus signal value at the status signal output of the termination unit.To retrieve information from the data bus subscribers, the local busmaster can generate a number of telegrams/data packets. In this case,the number of telegrams/data packets corresponds to the number ofcommunication-ready data bus subscribers displayed by the first counter.For example, if the first counter has indicated that there are N databus subscribers ready for communication, N telegrams/data packets arealso generated and sent on the local bus, wherein N is an integernatural number. Subsequently, the number of telegrams/data packets issent from the local bus master to the local bus. In each case one of thegenerated telegrams/data packets is intended for a communication-readydata bus subscriber and is assigned to the latter based on therespective relative position of the communication-ready data bussubscriber within the order. With this number of telegrams/data packets,it is then possible accordingly to retrieve information from the databus subscribers, wherein each telegram/data packet is assigned toexactly one communication-ready data bus subscriber, who uponrecognizing that the telegram/data packet is addressed to the data bussubscriber, fills the latter with information which can be read by thelocal bus master after passing through the local bus.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes, combinations,and modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

FIG. 1 is a schematic block diagram of an exemplary automation systemwith a programmable logic controller, a higher-level bus, and aplurality of exemplary data bus subscribers of the invention;

FIG. 2 is a schematic block diagram with at least two exemplary data bussubscribers according to the invention;

FIG. 3 is a schematic block diagram of the at least two exemplary databus subscribers as shown in FIG. 2 with a pull-up resistor at the statussignal input and a controller at the status signal input; and

FIG. 4 is a schematic voltage diagram of an exemplary data bussubscriber.

DETAILED DESCRIPTION

FIG. 1 shows a schematic block diagram of an automation system. It willbe understood by the person skilled in the art that the automationsystem shown is only an example and all the elements, modules,components, subscribers and units belonging to the automation system canbe configured differently but can nevertheless fulfill the basicfunctions described herein.

The automation system shown in FIG. 1 has a higher-level control 1,which can be realized for example with a programmable logic controlunit, PLC. Such a PLC 1 basically serves to control and regulate theprocess performed by the automation system. However, PLCs 1 inautomation systems today also take on more advanced functions, such asvisualization, alerts and recording of all data relating to the process,and as such, the PLC 1 functions as a human-machine interface. There arePLCs 1 with different performance levels, which have different resources(computing capacity, memory capacity, number and type of inputs andoutputs, and interfaces) that enable the PLC 1 to control and regulatethe process of the automation system. A PLC 1 usually has a modulardesign and is formed of individual components, each fulfilling adifferent task. Typically, a PLC 1 includes a central processing unit(with one or more main processors and memory modules) and multiplemodules with inputs and outputs. Such modular PLCs 1 can be easilyexpanded by adding modules. It depends on the complexity of the processand the complexity of the structure of the automation system as to whichmodules must be integrated in the PLC 1. In today's automation systems,the PLC 1 is also usually no longer an independent system, but insteadthe PLC 1 is connected via appropriate interfaces to the Internet orintranet. This means that the PLC 1 is part of a network through whichor from which the PLC 1 can obtain information, instructions,programming, etc. For example, through a connection to a computerlocated on the intranet or the Internet, the PLC 1 can obtaininformation about the materials supplied to the process, so that theprocess can be optimally controlled, for example, by knowing its numberor nature. It is also conceivable that the PLC 1 is controlled by a uservia access from the intranet or Internet. Thus, for example, a user canaccess the PLC 1 with the aid of a computer, also called a hostcomputer, and can check, change or correct its user programming.Accordingly, access to the PLC 1 is possible from one or more remotecontrol stations or control centers. If necessary, the host computerscan have visualization devices for displaying process sequences.

To control the process of the automation system, the PLC 1 is connectedto automation devices. In order to keep the wiring costs low, bussystems are used for these connections. In the exemplary embodimentshown in FIG. 1, the PLC 1 is connected to a local bus master 3 of asubordinate local bus system by means of a higher-level bus 2, which inthe exemplary embodiment shown here can be a field bus. However, notonly a local bus master 3 of a local bus can be connected to thehigher-level bus 2 as in the embodiment shown here, but also any othersubscribers which are designed for communication with the PLC 1.

In the exemplary embodiment shown here, the higher-level bus 2 isconnected to the local bus master 3. For this purpose, the local busmaster 3 has a first interface 4, which is designed such that it can beconnected to the higher-level bus 2. For this purpose, the interface 4can have, for example, a receptacle in the form of a socket and thehigher-level bus 2 can have a plug which can be received by the socket.In this case, the plug and the socket, for example, can be a modularplug and a modular socket, i.e., each core of the higher-level bus 2 iselectrically or optically connected to a connection in the modularsocket. However, the person skilled in the art also knows other ways inwhich an interface 4 can be designed so that the local bus master 3 canbe electrically or optically connected to the higher-level bus 2. Theperson skilled in the art is familiar with screw, turn, click or plugconnections, by means of which an electrical or optical connection canbe made. In most cases, a male plug is received by a female counterpart.This receptacle usually does not only establish the electrical oroptical connection, but also ensures that the two parts are mechanicallycoupled and can only be decoupled again with the application of acertain force. But it is also conceivable that the higher-level bus 2 ishardwired to the interface 4.

The local bus master 3 in the embodiment shown here has a further secondinterface to connect the local bus master 3 with the local bus, whereinthe local bus is designed as a ring bus 6 in the exemplary embodimentshown here. In this case, the second interface is divided into a firstpart 5 a and a second part 5 b. The first part 5 a of the secondinterface establishes the downlink in the ring bus 6 and the second part5 b of the second interface establishes the uplink in the ring bus 6.

In the embodiment shown here, the ring bus 6, the data transmissiondirection of which is shown with arrows in the exemplary embodimentshown in FIG. 1, includes the data bus subscribers 7 a, 7 b, . . . , 7n. In the exemplary embodiment shown here, these data bus subscribers 7a, 7 b, . . . , 7 n each have an interface 8 in order to receive datafrom an upstream or preceding data bus subscriber 7 a, 7 b, . . . , 7 n.In the case of data bus subscriber 7 a, the latter receives data fromthe upstream local bus master 3 via the interface 8. The data on thelocal bus 6 can also be referred to as local bus data. Further, in theembodiment shown here, the data bus subscribers 7 a, 7 b, . . . , 7 neach have an interface 9 to forward data to a downstream or subsequentdata bus subscriber 7 a, 7 b, . . . , 7 n. In the case of data bussubscriber 7 a, the latter sends data to the downstream data bussubscriber 7 b via the interface 9. The interfaces 8 and 9 serve topropagate data in the downlink direction of the ring bus 6, i.e., awayfrom the local bus master 3. Further, the data bus subscribers 7 a, 7 b,. . . , 7 n in this embodiment comprise interfaces 10 and 11 forpropagating data in the uplink direction of the ring bus 6, i.e., to thelocal bus master 3. In the case of the data bus subscriber 7 a,interface 10 is designed to receive data from the downstream orsubsequent data bus subscriber 7 b, and interface 11 is designed toforward data to the upstream or preceding data bus subscriber, here thelocal bus master 3. It can therefore also be said that the interfaces 9and 11 are transmitter interfaces, whereas the interfaces 8 and 10 arereceiver interfaces.

In the embodiment shown here, the connections of the interfaces and ofthe PLC 1 or the data bus participants 7 a, 7 b, . . . , 7 n arerealized by means of cables or printed circuit boards for direct orindirect contacting by electrical contacts. Another alternative is thatthe individual connections are made wirelessly, and the interfacesprovide the necessary conversions to the radio communication standardsused.

Even if the local bus master 3 and the individual data bus subscribers 7a, 7 b, . . . , 7 n in the embodiment shown here are shown spaced fromeach other, i.e., the local bus master 3 is arranged decentrally fromthe data bus subscribers 7 a, 7 b, . . . , 7 n, the person skilled inthe art is aware that the data bus subscribers 7 a, 7 b, . . . , 7 n andthe local bus master 3—which also represents a data bus subscriber ofthe ring bus 6—can also be connected directly to one another. In thiscase, for example, contacts of the one data bus subscriber can engage incorresponding receptacles or receptacle contacts of a directly adjacentdata bus subscriber so as to establish an electrical connection betweenthe data bus subscribers such that data can be transmitted in thedownlink and uplink direction. For example, the data bus subscribers 7a, 7 b, . . . , 7 n may have receptacles at the side facing away fromthe master, and contacts on the side facing the master. If the data bussubscribers 7 a, 7 b, . . . , 7 n are then lined up correspondingly, thecontacts of the one data bus subscriber 7 a, 7 b, . . . , 7 n eachengage in the receptacles of the other data bus subscriber 7 a, 7 b, . .. , 7 n and an electrical connection can be created. The local busmaster 3 then has corresponding contacts on the side, which engage inthe receptacles of the first data bus subscriber 7 a so as to produce anelectrical connection between the interfaces 5 a and 8 or the interfaces5 b and 11. The person skilled in the art is also aware of other ways,e.g., pressure contacts, knife/fork contacts, of how two, directlyadjoining data bus subscribers 7 a, 7 b, . . . , 7 n can establish anelectrical or optical connection.

In the case that the data bus subscribers 7 a, 7 b, . . . , 7 n and thelocal bus master 3 are to be connected directly to one another, they canalso have mechanical receptacles or mechanical fastener by means ofwhich the individual data bus subscribers 7 a, 7 b, . . . , 7 n and thelocal bus master 3 can be connected to each other. Here, for example, adata bus subscriber 7 a, 7 b, . . . , 7 n can comprise a projection onone side and an undercut on the other. If the data bus subscribers 7 a,7 b, . . . , 7 n are then lined up, a projection engages in an undercutof the other data bus subscriber 7 a, 7 b, . . . , 7 n, such that amechanical coupling is produced. For the simple juxtaposition of thedata bus subscribers 7 a, 7 b, . . . , 7 n, these can also be arrangedon a common receptacle, for example a DIN rail. For fastening on the DINrail, the data bus subscribers 7 a, 7 b, . . . , 7 n can have acorresponding fastener. Alternatively, or additionally, the data bussubscribers 7 a, 7 b, . . . , 7 n can also have, for example, releasablyconnectable fastener by means of which the data bus subscribers 7 a, 7b, . . . , 7 n can be secured to either the rail or another receptacle.For this purpose, the releasably connectable fastener can beinterchangeable and a corresponding fastener for the desired receptaclecan be connected to the data bus subscribers 7 a, 7 b, . . . , 7 n, suchthat these can be attached to the desired receptacle.

Further, the data bus subscribers 7 a, 7 b, . . . , 7 n in theembodiment shown in FIG. 1 also comprise a processing unit 12. Thisprocessing unit 12 can be an arithmetic logic unit or another type ofarithmetic unit with which data can be processed. The processing unit 12is preferably an integral part of the data bus subscriber 7 a, 7 b, . .. , 7 n in order to ensure a particularly fast and time-synchronizedprocessing of the data.

The processing unit 12 may also be referred to as the whole circuit ofthe data bus subscriber. This means, the processing unit 12 receivesdata via the inputs 8 and 10 and outputs data at the outputs 9 and 11.In addition, the processing unit 12 can receive or output data from/tothe input/outputs 13 and 14, respectively. Furthermore, the processingunit 12 has access to a memory of the data bus subscriber 7 a, 7 b, . .. , 7 n in which, for example, data, process data, or instruction listsare stored.

The processing unit 12 may be configured to process received data and tooutput data. Data to be processed can be received either from anupstream data bus subscriber or from inputs 13 of the data bussubscriber 7 a, 7 b, . . . , 7 n. In this case, the inputs 13 of thedata bus subscriber 7 a, 7 b, . . . , 7 n can be connected to sensors15, which transmit, for example, measurement data, status data, etc.Processed data can be output either to a downstream data bus subscriberor at outputs 14 of the data bus subscriber 7 a, 7 b, . . . , 7 n. Theoutputs 14 of the data bus subscriber 7 a, 7 b, . . . , 7 n can beconnected to actuators 16, which, for example, perform a particularaction by means of the data addressed to them. If processing of the datais also to take place in the uplink direction, data can also be receivedby a downstream data bus subscriber 7 a, 7 b, . . . , 7 n and processeddata can be transmitted to an upstream data bus subscriber 7 a, 7 b, . .. , 7 n.

For the sake of simplicity, in the exemplary embodiment shown here, thedata bus subscribers 7 a, 7 b, . . . , 7 n are shown with only one input13 and one output 14, and only data bus subscriber 7 b is connected tosensor 15 and actuator 16. However, it is clear to the person skilled inthe art that the data bus subscribers 7 a, 7 b, . . . , 7 n may comprisea plurality of inputs and outputs 13 and 14 and can be connected to aplurality of different sensors 15 and actuators 16. In this case, thefeature characterizing the sensors 15 is that the sensors 15 receivedata or signals and transmit these to the data bus subscriber 7 a, 7 b,. . . , 7 n, whereas actuators 16 receive data or signals from the databus subscribers 7 a, 7 b, . . . , 7 n and perform an action based onthese data or signals.

Alternatively, the interfaces 8, 9, 10 and 11 can be integrated in onemodular unit and the data bus subscribers 7 a, 7 b, . . . , 7 n can beplugged onto this modular unit. The modular units can also be referredto as basic elements of the ring bus 6. The ring bus infrastructure isconstructed by the modular units and the data bus subscribers 7 a, 7 b,. . . , 7 n are interchangeable, so that the ring bus 6 can beconstructed with any data bus subscriber 7 a, 7 b, . . . , 7 n. With thehelp of the modular units, it is also ensured that even if a data bussubscriber 7 a, 7 b, . . . , 7 n is removed, the communication betweenthe remaining data bus subscribers 7 a, 7 b, . . . , 7 n is notinterrupted because communication takes place via the remaining modularunits.

The data bus subscribers 7 a, 7 b, . . . , 7 n shown in this embodimentare also often referred to as I/O modules on account of their inputs andoutputs 13, 14 that can be connected to sensors 15 or actuators 16. Evenif the data bus subscribers 7 a, 7 b, . . . , 7 n shown here in theexemplary embodiment are presented as spatially separated from thesensors 15 or actuators 16, the sensors 15 or actuators 16 can also beintegrated in the I/O module.

The ring bus 6 shown in the embodiment shown here is based on a cycleframe communication. Here, the local bus master 3 generates, forexample, telegrams/data packets in a cycle frame which have a header, aprocess data part or an information data part and a checksum part. Theone or more cycle frames are sent from the local bus master 3 in thedownlink direction to the first data bus subscriber 7 a of the ring bus6. The latter receives a first part of the cycle frame via the interface8. Such a part of the cycle frame is also referred to below as a pieceor unit or symbol. The data bus subscriber 7 a then carries out aprocessing of the part, and then forwards the part to the next data bussubscriber 7 b via interface 9; preferably simultaneously, the firstdata bus subscriber 7 a receives a second part of the cycle frame, etc.The size of the parts of the cycle frame, i.e., the split of the cycleframe, depends on the capacity of the data bus subscribers 7 a, 7 b, . .. , 7 n. For example, at the same time, a fixed number of bits, forexample 8 bits of the cycle frame, can be present at the data bussubscriber 7 a, 7 b, . . . , 7 n for processing.

The cycle frame accordingly passes unit-by-unit, piecewise, or in parts,for example in parts or symbols of 8 bits through the data bussubscribers 7 a, 7 b, . . . , 7 n and the termination unit 17 x shownhere. The part of the cycle frame which has been processed by the lastdata bus subscriber, data bus subscriber 7 n in the embodiment shownhere, is routed from the termination unit 17 x from the interface 9 ofthe data bus subscriber 7 n to its interface 10, so that this part ofthe cycle frame can again pass through the ring bus 6 in the uplinkdirection, i.e. in the direction of the local bus master 3. In theprocess, all parts of the cycle frame again pass through the respectivedata bus subscribers 7 y, 7 b, . . . , 7 n of the ring bus 6.Alternatively, the parts of the cycle frame can also be routed past alldata bus subscribers 7 a, 7 b, . . . , 7 n to the local bus master 3with the aid of a bypass line.

If the parts of the cycle frame are again routed in the uplink directionthrough all data bus subscribers 7 a, 7 b, . . . , 7 n, they can bere-processed by the data bus subscribers 7 a, 7 b, . . . , 7 n so thatthe cycle frame can be processed twice, once in the downlink directionto the last data bus subscriber 7 n and once in the uplink direction tothe local bus master 3. For example, processing may take place upstreamby means of a signal refresh and/or phase shift.

In the processing of the cycle frame in the downlink direction, i.e.,away from the local bus master 3, or in the uplink direction, i.e.,towards the local bus master 3, the processing is accomplished by meansof instruction lists, wherein the instruction lists contain sets ofinstructions which can be executed by the processing unit 12 of the databus subscribers 7 a, 7 b, . . . , 7 n. The instruction lists themselvescan be sent to the individual data bus subscribers 7 a, 7 b, . . . , 7 nin an initialization phase by the local bus master 3 or, advantageously,to the data bus subscribers 7 a, 7 b, . . . , 7 n during the ongoingcommunication, so that programming of the data bus subscribers 7 a, 7 b,. . . , 7 n takes place without interrupting the communication.

Which of the instruction lists the data bus subscribers 7 a, 7 b, . . ., 7 n should use can be indicated to the data bus subscribers 7 a, 7 b,. . . , 7 n on the basis of an instruction list index. This instructionlist index informs the data bus subscribers which stored instructionlist to use. An instruction list index is thus assigned to aninstruction list or vice versa, so that the instruction list to be usedcan be identified with the aid of the instruction list index. For thispurpose, the instruction list index preferably has a value which isassigned to an instruction list, for example, the value indicates aspecific instruction list or its memory location. For this purpose, thevalue itself may be the memory address where the instruction list isstored or where at least a first instruction of the instruction list isstored. Alternatively, or additionally, the value can also indicate amemory area in which the corresponding instruction list is stored. Theaforementioned cases can also be referred to as a direct assignment. Thevalue of the instruction list index, however, can also be used, forexample, as input of a look-up table (LUT). In this case, the value ofthe instruction list index is the input value of the conversion table.The output value of the conversion table can be the memory address ofthe first instruction in the associated instruction list or canotherwise identify the instruction list. The conversion table can bestored in terms of software technology or hardware in the form of, forexample, logics and can indicate a one-to-one conversion from an inputvalue to an output value, wherein the output value gives an indicationof the instruction list to be used. It depends on the conversion tableas to how a relationship between the instruction list index and theinstruction list is established. When using a conversion table, it isalso possible to speak of an indirect assignment. However, in both thedirect and indirect assignment, the instruction list to be used, i.e.,found, by the data bus subscriber is uniquely identifiable via theinstruction list index. The instruction list index can be added to thecycle frames prior to processing the process data so that the data bussubscribers 7 a, 7 b, . . . , 7 n can use the corresponding instructionlist according to the order of the process data in the cycle frame. Theinstruction lists contain instructions that are adapted to the order ofthe process data in the cycle frame. In this case, the instruction listscan, for example, have a “SKIP” instruction for process data which isnot addressed to the data bus subscribers 7 a, 7 b, . . . , 7 n, i.e.,instruct the data bus subscriber 7 a, 7 b, . . . , 7 n to skip thecorresponding part of the cycle frame, whereas the instruction list forprocess data directed to the data bus subscriber 7 a, 7 b, . . . , 7 nmay have corresponding instructions for processing the process data. Theprocessing of the process data can thus be decoupled from the actualposition of the process data in the cycle frame, since the data bussubscribers are adapted to the order of the process data in the cycleframe with the aid of the instruction lists.

As already described above, the embodiment of FIG. 1 shows thetermination unit 17 x which is connected to data bus subscriber 7 n. Thetermination unit 17 x connects the interfaces 9 and 10 of the data bussubscriber 7 n, so that the downlinked cycle frame is again sent inreverse order to the local bus master 3, i.e., in the uplink direction,by all data bus subscribers 7 a, 7 b, . . . , 7 n. The cycle frame orthe parts of the cycle frame can be re-processed by the data bussubscribers 7 a, 7 b, . . . , 7 n in the uplink direction or can belooped through the data bus subscribers 7 a, 7 b, . . . , 7 n withoutfurther processing. The termination unit 17 x further provides a statussignal at its status signal output 18, which has a previously definedstatus signal value. This status signal or the status signal value ofthe termination unit 17 x is received at the data bus subscriber 7 n atits status signal input 20 n. If the data bus subscriber 7 n is ready tocommunicate, i.e., if the data bus subscriber 7 n is clock-synchronizedwith the local bus 6 and the processing unit 12 n is capable ofprocessing parts of the cycle frame, then the data bus subscriber 7 nchanges its status signal value at its status signal output 21 n to astatus signal value indicating the communication readiness. In thiscase, the status signal value may, for example, assume a low statebefore the communication readiness and assume a high state after thecommunication readiness has been established. The status signal valuecan therefore be set from a logical zero to a logical one. However, theperson skilled in the art is aware that other states or status signalvalues can also be defined which indicate the readiness to communicate.It is only important that there is a change in the status signal valueat the status signal output when communication readiness has beenestablished. As an alternative to the termination unit 17 x shown here,the data bus subscriber 7 n itself can also assume the functionality ofthe termination unit 17 x. That is, if the data bus subscriber 7 ndetermines that it is the last data bus subscriber of the local bus 6and determines that no termination unit 17 x is connected, then the databus subscriber 7 n itself can again send the parts of the cycle framereceived from the downlink direction in the uplink direction. The databus subscriber 7 n can accordingly be adapted in order to establish aconnection between the interface 9 and the interface 10. Furthermore, inthis case, the data bus subscriber 7 n can be adapted to perform astatus signal value change at its status signal output 21 n afterestablishing its readiness for communication, even if no status signalvalue is present at its status signal input 20 n.

With the change of the status signal value at the status signal output21 n, a status signal value is present at data bus subscriber 7 b at itsstatus signal input 20 b, which indicates that the data bus subscriber 7n is ready to communicate. Because this status signal value is the samestatus signal value output by the termination unit 17 x, if this ispresent, it can also be said that the status signal is routed throughthe data bus subscriber 7 n when it is ready to communicate.Accordingly, the status signal is also routed through the other data bussubscribers 7 a, 7 b when these are ready to communicate. If all thedata bus subscribers 7 a, 7 b, . . . , 7 n are ready to communicate,then it can be said that a status signal line exists between thetermination unit 17 x with its status signal output 18 and the local busmaster 3 with its status signal input 5 c. This status signal line mayphysically be a line which, by means of switches or couplers,establishes an electrical or optical connection between the statussignal output 18 and the status signal input 5 c. This status signalline can pass the signal originating from the status signal output 18 tothe status signal input 5 c. As already described above, this signalline can also represent only a connection between the status signaloutput 21 n of the last data bus subscriber 7 n and the status signalinput 5 c, namely in the case in which no termination unit 17 x ispresent. As an alternative to establishing a physical connection, thesignal line can also only exist on an organizational level and each databus subscriber 7 a, 7 b, . . . , 7 n acts as a repeater of the statussignal value signal present at its status signal input 20 a, 20 b, . . ., 20 n and generates the same status signal value at its status signaloutput 21 a, 21 b, . . . , 21 n when the data bus subscriber 7 a, 7 b, .. . , 7 n is ready to communicate and the status signal value at thestatus signal input 20 a, 20 b, . . . , 20 n indicates that thedownstream data bus subscribers are also ready to communicate. In thiscase, it is also possible to speak of a status signal line, because inthis case too, the status signal value of the status signal output 18 ofthe termination unit 17 x or the status signal value of the statussignal output 21 n of the last data bus subscriber 7 n is forwarded tothe status signal input 5 c of the local bus master 3, even if there isno direct physical connection between the status signal output 18 of thetermination unit 17 x or the status signal output 21 n of the data bussubscriber 7 n and the status signal input 5 c of the local bus master3. In the case of a solely organizational status signal line, it canalso be said that there is only one physical connection between in eachcase two data bus subscribers 7 a, 7 b, . . . , 7 n, but there is nophysical connection between all the data bus subscribers 7 a, 7 b, . . ., 7 n.

Even if the status signal line in the exemplary embodiment shown here issimilar to the local bus 6, the status signal line is separate from thelocal bus 6. That is, the status signal is not propagated via a bus linewhich is designed to carry data. In FIG. 2, this separation is shown indetail.

FIG. 2 shows a schematic block diagram with two exemplary data bussubscribers7 b and 7 n from the local bus 6 corresponding to FIG. 1. Thedata bus subscribers 7 b and 7 n are connected in the example shown hereto base elements 17 b and 17 n, which establish the infrastructure forthe local bus 6 and the status signal line 19. The base elements 17 band 17 n have switchable connections for the downlink direction anduplink direction of data. For this purpose, for example, the baseelement 17 b contains the switches 8 b′, 9 b′, 11 b′ and 10 b′. Theswitches 8 b′ and 9 b′ may either route the downlink direction of datato the data bus subscriber 7 b plugged on the base element 17 b or mayroute it through the base element 17 b if, for example, no data bussubscriber 7 b is present. The uplink direction of data may also berouted with the switches 10 b′ and 11 b′ either through the data bussubscriber 7 b plugged onto the base element 17 b or may be routedthrough the base element 17 b if, for example, no data bus subscriber 7b is present. The designation of the switches is accompanied by thedesignation of the respective interfaces 8 b, 9 b, 10 b, and 11 b on thedata bus subscriber 7 b. Thus, the switch 8 b′ of the base element 17 bswitches the data in the downlink direction on the interface 8 b; theswitch 9 b′ switches the data coming from the interface 9 b of the databus subscriber 7 b to the downstream base element 17 n. The switch 10 b′switches the data coming in the uplink direction from the base element17 n to the interface 10 b of the data bus subscriber 17 b, and theswitch 11 b′ switches the data coming from the interface 11 b of thedata bus subscriber 17 b to the upstream base element 17 a here. Theswitches 8 b′, 9 b′, 10 b′, and 11 b′ can be switched via a signal inputat the input 26 b. This signal input may be a mechanical, electrical oroptical signal input. For example, the switches 8 b′, 9 b′, 10 b′, and11 b′ may switch when a mechanical connection is made between the baseelement 7 b and the data bus subscriber 7 b. However, it is alsoconceivable that the switches 8 b′, 9 b′, 10 b′, and 11 b′ only switchwhen there is an electrical or optical signal input generated by thedata bus subscriber 7 a. In the embodiment shown here, the base elements17 b and 17 n are respectively connected to the data bus subscribers 7band 7n. In this case, the switches 8 b′, 9 b′, 10 b′, 11 b′ and 8 n′, 9n′, 10 n′, 11 n′ are switched such that the downlink direction anduplink direction of the data are each routed in such a way that theseare routed to the respective interfaces 8 b, 9 b, 10 b, 11 b and 8 n, 9n, 10 n, 11 n of the data bus subscribers 7 b and 7 n.

Further, the data bus subscribers 7 b and 7 n themselves have switches22 b, 23 b and 22 n, 23 n. With the help of these switches 22 b, 23 band 22 n, 23 n, data can bypass the respective processing units 12b and12n in the downlink and in the uplink direction, for example, if theseare not yet able to process the corresponding data.

In the embodiment shown here, the processing unit 12 n of the data bussubscriber 7 n is already able to process data, whereas the processingunit 12 b of the data bus subscriber 7 b is not yet able to processdata.

In the data bus subscriber 7 n, the switch 22 n is switched such that itconnects the interface 8 n to the processing unit 12 n instead ofconnecting the interface 8 n directly to the interface 9 n, such thatdata can be processed in the downlink direction by the processing unit12 n. In the uplink direction, the switch 23 n is connected such thatthis connects the interface 10 n with the processing unit 12 n insteadof directly connecting the interface 10 n with the interface 11 n, suchthat data can be processed in the uplink direction by the processingunit 12 n.

In the embodiment shown here, the processing unit 12 b of the data bussubscriber 7 b is not yet able to process data. Accordingly, theswitches 22 b and 23 b are switched such that the data bypasses theprocessing unit 12 b. In the embodiment shown here, this is done by theinterface 8 b being connected directly to the interface 9 b and theinterface 10 b being connected directly to the interface 11 b. As soonas the processing unit 12 b is able to process data, the switches 22 band 23 b are switched and are switched to the switching statecorresponding to the data bus subscriber 7 n, so that data can beprocessed by the processing unit 12 b.

The person skilled in the art is aware that even if a re-processing ofthe data is shown here in the uplink direction and the latter are routedthrough the processing units 12 b and 12 n, provided these are ready,this does not have to be the case. If no further processing is to takeplace in the uplink direction, then the switches 23 b and 23 n canalways be switched in such a way that the interfaces 10 b and 11 b or 10n and 11 n are connected to one another. It is also conceivable that inthis case the switches 23 b and 23 n are omitted and that anon-switchable connection exists between the interfaces 10 b and 11 b or10 n and 11 n. It is also conceivable that in this case the switches 10b′, 11 b′ and 10 n′, 11 n′ of the base elements 17 b and 17 n areomitted and that the uplink direction is routed directly through thebase elements.

Furthermore, the base elements 17 b and 17 n include a part of thesignal line 19, namely sections 19 b′ and 19 n′. The signal line startsat the termination unit 17 x. The termination unit 17 x applies a statussignal value to its status signal output 18, status signal value GND inthe exemplary embodiment shown here; the latter is routed via the baseelement 17 n to the status signal input 20 n of the data bus subscriber7 n. Since the data bus subscriber 7 n is already ready to communicate,i.e., it is clock-synchronized to the data on the local bus 6 and theprocessing unit 12 n is able to receive and process data such that theswitches 22 n and 23 n no longer provide a bridging of the processingunit 12 n, the status signal value coming from the status signal input20 n is linked with a discrete value 25 n at the logical link unit 24 n.This discrete value 25 n is generated or set to a specific value by theprocessing unit 12 n as soon as the processing unit 12 n is ready tocommunicate. The logical link unit 24 n is adapted to establish alogical linkage of the status signal value of the status signal input 20n and the discrete value 25 n. In this case, the linkage is accomplishedsuch that only when the status signal value of the status signal input20 n indicates that the downstream data bus subscribers are ready tocommunicate—specified here by the termination unit 17 x—and the discretevalue 25 n indicates that the processing unit 12 n is ready forcommunication, then the status signal value of the status signal output21 n of the data bus subscriber 7 n is set to the same status signalvalue as the status signal value of the status signal input 20 n of thedata bus subscriber 7 n. This status signal value applied to the statussignal output 21 n of the data bus subscriber 7 n is then forwarded tothe upstream data bus subscriber 7 b via the base elements 17 n and 17b.

In the exemplary embodiment shown here, the data bus subscriber 7 breceives the status signal value of the status signal output 21 n of thedata bus subscriber 7 n at its status signal input 20 b. However, sincethe processing unit 12 b of the data bus subscriber 7 b is not yet readyto communicate, and this is still bypassed by means of the switches 22 band 23 b, the discrete value 25 b is not yet set to a value indicativeof communication readiness. That is to say, the logical link unit 24 bstill outputs a status signal value at the status signal output 21 b ofthe data bus subscriber 7 b indicating that the data bus subscriber 7 bor its processing unit 12 b is not yet ready to communicate. That is, astatus signal value is output which differs from the status signal valuereceived at the status signal input 20 b of the data bus subscriber 7 b.Only when the data bus subscriber 7 b or its processing unit 12 b isready for communication is the discrete value 25 b set to a value whichcauses the logical link unit 24 b to output a status signal value at thestatus signal output 21 b of the data bus subscriber 7 b, whichcorresponds to the status signal value of the status signal input 20 bof the data bus subscriber 7 b. That is to say only when all the databus subscribers 7 a, 7 b, . . . , 7 n are ready to communicate is thesame status signal value received via the status signal input 5 c of thelocal bus master 3 which was generated by the termination unit 17 x orthe last data bus subscriber 7 n. The local bus master 3 is then awarethat the local bus 6 is complete and that all data bus subscribers 7 a,7 b, . . . , 7 n are ready to communicate. The local bus master 3 canthen send telegrams/data packets to the local bus 6, which serve toidentify the respective connected data bus subscribers. Already duringthe initialization of the local bus 6, i.e., before the local bus master3 detects a change to the status signal input 5 c, can the local busmaster 3 send such telegrams/data packets to the local bus 6 because thedata bus subscribers 7 a, 7 b, . . . , 7 n successively become ready tocommunicate. Since the data bus subscribers 7 a, 7 b, . . . , 7 n notyet ready to communicate are bypassed, the others can alreadycommunicate with the local bus master 3 to exchange identityinformation. If the status signal value changes at the status signalinput 5 c of the local bus master 3, however, the latter has knowledgethat the local bus 6 is complete and can send the correspondingtelegrams/data packets one last time in order to identify thenot-yet-identified data bus subscribers 7 a, 7 b, . . . , 7 n.

If no data bus subscriber 7 a, 7 b, . . . , 7 n is arranged on one ofthe base elements 17 a, . . . , 17 n, a cap can be arranged on thelatter, which bridges the signal line 19 for this base element 17 a, . .. , 17 n, i.e., establishes a conductive connection. Otherwise, in a notfully occupied local bus 6, it would never be displayed that all databus subscribers 17 a, . . . , 17 n are ready to communicate. Instead ofa cap, the base elements 17 a, 17 n can also be designed toindependently provide a bridging of the signal line 19 if no data bussubscriber 7 a, 7 b, . . . , 7 n is present. For this purpose, the baseelements 17 a, . . . , 17 n recognize the presence of a data bussubscriber 7 a, 7 b, . . . , 7 n and switches the bridging accordingly.The bridging can be switched mechanically or electrically. For example,when plugging the data bus subscriber 7 a, 7 b, . . . , 7 n, the lattercan activate a mechanism in or at the respective base element 17 a, . .. , 17 n with a geometry arranged on data bus subscriber 7 a, 7 b, . . ., 7 n, such that the bridging is canceled and the signal line 19 isrouted through the data bus subscriber 7 a, 7 b, . . . , 7 n.

FIG. 3 shows a schematic block diagram of the two exemplary data bussubscribers 7 b and 7 n as shown in FIG. 2. In this case, the data bussubscriber 7 b has a pull-up resistor 27 at its status signal input 20b. This ensures that when the status signal output 21 n of the data bussubscriber 7 n is of high impedance, a logical one is applied to thestatus signal input 20 b. Only when changing the resistance value at thestatus signal output 21 n can the logic state change, namely to thestate which is provided at the status signal output 21 n of the data bussubscriber 7 n. This provided state corresponds to the status signal ofthe data bus subscriber 7 n or its status signal value. If the data bussubscriber 7 n is ready to communicate, then it provides a status signalat its status signal output 21 n, which indicates that the data bussubscriber 7 n is ready to communicate. In this case, the data bussubscriber 7 n changes the resistance value of the status signal output21 n. If this was initially of high impedance as compared to the pull-upresistor 27 and is changed to low impedance, then the logic state givenby the pull-up resistor is no longer present at the status signal input20 b, but instead the logic state of the status signal of the statussignal output 21 n. For example, at a first resistance value of thestatus signal output 21 n, the pull-up resistor 27 can ensure that thestatus signal input 20 b has a logical state of one, whereas the statussignal has a logical zero. However, since the resistance value of thestatus signal output 21 n is of even higher impedance than theresistance of the pull-up resistor 27, the status signal is not receivedat the status signal input 20 b. Only when the resistance value of thestatus signal output 21 n changes to low impedance as compared to thepull-up resistor 27 is the status signal received at the status signalinput 20 b and does the logic state change from one to zero. In theembodiment shown here, the resistance change is provided by means of twoseparate MOSFETs, as can be seen in the magnification. These eachconnect either a high or low resistance to the status signal output 21n, wherein the resistance values are adapted to the pull-up resistor 27.However, it is well known to the person skilled in the art that thereare other possibilities to make a corresponding change in resistance.The change in resistance ensures that there are no undefined states atthe status signal inputs at any time. This is particularly important inthe chain-based forwarding of the status signal because an error on onedata bus subscriber in the chain could cause the upstream data bussubscribers to provide a false status signal value at their statussignal outputs.

FIG. 4 shows a schematic voltage diagram of two data bus subscribers 7 band 7 n. In this case, the curve 28 shows the voltage characteristic atthe status signal input 20 b of the data bus subscriber 7 b, while thecurve 29 shows the supply voltage characteristic of the data bussubscriber 7 n. When the supply voltage of the data bus subscriber 7 nis switched on, an increase is initially evident over a certain time—seesection a—until the supply voltage has assumed the value for normaloperation. During this time, the voltage of the status signal input 20 bis held in the logical state one by the pull-up resistor 27. This is dueto the fact that the data bus subscriber 7 n has not yet changed theresistance value of its status signal output 21 n at this time. Withoutthis maintenance of the resistance value, the voltage of the statussignal input 20 b would already assume an undesired state when thesupply voltage increases. However, the maintenance of the resistancevalue of the status signal output 21 n ensures that the logical state ofthe status signal input 20 b remains constant. Only when the data bussubscriber 7 n is ready to communicate is there a change in resistanceof the status signal output 21 n of the data bus subscriber 7 n, whichleads to a change in the voltage at the status signal input 20 b of thedata bus subscriber 7 b, i.e., to a change in the logical state—seesection b. In this way, a status signal can be conditionally forwardfrom one data bus subscriber 7 a, 7 b, . . . , 7 n to the other withoutcausing undesirable states.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

What is claimed is:
 1. A data bus subscriber connected to a local bus,in particular a ring bus, the data bus subscriber comprising: a statussignal input for receiving a first status signal value from a downstreamdata bus subscriber or a termination unit; and a status signal outputfor providing a second status signal value to an upstream data bussubscriber or to a local bus master, wherein the data bus subscriber isadapted to provide the second status signal value based on a logicallinkage of a communication readiness of the data bus subscriber and thefirst status signal value.
 2. The data bus subscriber according to claim1, wherein the data bus subscriber is ready to communicate as soon asthe data bus subscriber is clock-synchronized on the local bus.
 3. Thedata bus subscriber according to claim 2, further comprising a bypassconnection for routing telegrams past the processing unit as long as thedata bus subscriber is not ready to communicate or the processing unitcannot yet process the telegrams.
 4. The data bus subscriber accordingto claim 1, wherein the first and the second status signal value assumesat least two discrete states or a low and high states.
 5. The data bussubscriber according to claim 1, further comprising a controller forchanging a resistance value at the status signal output for changing thesecond status signal value.
 6. The data bus subscriber according toclaim 5, wherein the controller is adapted to change or reduce theresistance value at the status signal output from a first resistancevalue to a second resistance value.
 7. The data bus subscriber accordingto claim 1, wherein the logical link is configured such that the firststatus signal value and the second status signal value assume a samediscrete state when the data bus subscriber is ready to communicate. 8.The data bus subscriber according to claim 7, wherein the logical linkis configured to set the second status signal value to a fixed state ifthe data bus subscriber is not ready to communicate.
 9. A method foroperating a data bus subscriber connected to a local bus, in particulara ring bus, the method comprising: receiving a first status signal valueat the data bus subscriber from a downstream data bus subscriber or froma termination unit; and providing a second status signal value to anupstream data bus subscriber or a to local bus master by the data bussubscriber, the step of providing comprising logically linking acommunication readiness of the data bus subscriber and the first statussignal value to provide the second status signal value.
 10. A local bus,in particular a ring bus, the local bus comprising: at least one databus subscriber having a status signal input and a status signal output;and a termination unit having a status signal output, wherein the statussignal output of the termination unit and the status signal input of thedata bus subscriber are connected, wherein the termination unit isadapted to provide a first status signal value to the status signaloutput of the termination unit that is received at the status signalinput of the data bus subscriber, and wherein the data bus subscriber isadapted to provide a second status signal value at the status signaloutput of the data bus subscriber based on the communication readinessof the data bus subscriber and the received first status signal value atthe status signal input of the data bus subscriber.
 11. The local busaccording to claim 10, further comprising: a local bus master comprisinga status signal input adapted to receive the second status signal valueprovided at the status signal output of the at least one data bussubscriber.
 12. The local bus according to claim 10, wherein the localbus master sends data packets to the local bus for counting the data bussubscribers that are ready to communicate.
 13. The local bus accordingto claim 12, wherein the local bus master is adapted to send a datapacket for retrieving information to each counted communication-readydata bus subscriber to the local bus.
 14. The local bus according claim13, wherein the local bus master is adapted to repeat the counting ofthe data bus subscribers and the transmission of the number of datapackets at least as long as a state of the second status signal value atthe status signal input of the local bus master does not correspond to astate of the first status signal value at the status signal output ofthe termination unit.